When molding materials comprising continuous fiber-reinforced thermoplastic resins are classified on the basis of the impregnation state of the resins, the materials are broadly grouped as follows: molding materials in sheet forms, namely molding materials of so-called completely impregnated type, wherein the resins are once melted and completely impregnated in between the reinforcing continuous fibers; and molding materials wherein the resins are present in their non-melted state, namely molding materials of non-melted type.
Although the former molding materials of completely impregnated type are not drapable because the resins therein form a matrix layer, the latter molding materials of non-melted type are drapable because the non-melted resins are present between the reinforcing continuous fibers.
In the molding materials of the completely impregnated type, the resins are preliminarily impregnated completely in between the reinforcing continuous fibers. Therefore, no need exists to re-impregnate the resins in a molding process. Thus, such molding materials can produce molded articles in a relatively short period by selecting and effecting an appropriate heating process and an appropriate molding pressure during molding. Hence, the materials have outstandingly good high-speed moldability and are a convenient material form for high productivity. For the production of a molded article of a complex shape such molding materials of the completely impregnated type may be used; however, the molded materials cause difficulty in lay-up due to not being drapable. Therefore, a molded article is then likely to be limited to a relatively simple shape.
In contrast, the latter molding materials of the non-melted type are provided with drape properties so as to overcome such a problem. The latter molding materials are broadly divided into molding materials produced by forming a thermoplastic resin into a fiber and then mixing the fiber with a reinforcing continuous fiber (molding materials by so-called fiber method) and molding materials produced by pulverizing a thermoplastic resin and then depositing the resulting pulverized resin onto a reinforcing continuous fiber (molding materials by so-called powder method).
The molding materials produced by the fiber method include hybrid yarn (produced by twisting a reinforcing continuous fiber and a resin fiber together), commingled yarn (comprising a reinforcing continuous fiber and a resin fiber, which are preliminarily mixed together at a monofilament level), and co-woven (woven fabric using hybrid yarn and commingled yarn). By the fiber method, resins which can not be formed into fiber cannot be used as matrix resins, and therefore, the resin types to be used by the method are limited to some extent.
The molding materials produced by the powder method are advantageous in that almost all resins can be pulverized finely so a wide variety of resins may be selected. The powder method for producing molding materials includes a method comprising passing a reinforcing continuous fiber through a fluid bed of a thermoplastic resin powder, a method comprising depositing a thermoplastic resin powder onto a reinforcing fiber in an electrostatic manner, and a method comprising passing a reinforcing continuous fiber through a bath suspending a thermoplastic resin powder. The molding materials produced by these methods are drapable, but because the thermoplastic resin powder is only physically interposed in between the reinforcing continuous fibers in the materials, the thermoplastic resin powder is readily dissociated from the reinforcing continuous fiber by an external force. Additionally, the reinforcing continuous fiber is also readily split by an external force. Thus, the molding materials are problematic in terms of handleability.
To overcome these problems, the following processes described in the items (1) to (4) are already known.
(1) A process of coating the surface of a thermoplastic resin powder-deposited reinforcing continuous fiber bundle in a tube shape with a melted resin, comprising continuously passing the bundle through the melted resin by means of a crosshead extruder (see Specification of France Patent Application No. 2548084 (Al)).
(2) A process of heating a thermoplastic resin powder-deposited reinforcing fiber bundle (fiber bundle) to semi-melt the resin to deposit the resin onto the reinforcing fiber (see J. Thermoplastic Composite Materials, Vol. 3, October 1990, pp. 325-354, "Manufacture of Powder-Impregnated Thermoplastic Composites").
(3) A process of winding a thermoplastic resin fiber over a reinforcing fiber bundle deposited with a thermoplastic resin powder ("VARIABLES AFFECTING THE PHYSICAL PROPERTIES OF CONSOLIDATED FLEXIBLE POWDER-COATED TOWPREGS," D. W. Holty et al., submitted for publication at the 38th International SAMPE Symposium: May 10-13, 1993).
(4) A process of dispersing a thermoplastic resin powder into an aqueous medium containing, dissolved therein a water-soluble polymeric binder, and impregnating a reinforcing fiber bundle with the resulting viscosity-increased dispersion/solution to prepare a molding material containing the aqueous medium in the amount of 10 to 70% by weight (Japanese Patent Marking No. Hei 1-501233 (1989) and Japanese Patent Laid-open No. Hei 1-228806 (1989)).
The techniques for providing drape, described as (1) to (4) above, have the following drawbacks. More specifically, the method in the item (1) is disadvantageous in that the resin is at such a higher level in the resulting molding material that it is difficult to elevate the fiber content therein and additionally in that a resin-enriched layer is formed in between the reinforcing continuous fibers adjacent to each other after molding, so that portions with no presence of fiber therein are formed in part. The cross-section of the molding material is of an elliptical shape, so the material is so bulky that molds for molding such a material are designed with much difficulty, which limits the molding method.
According to the method in the item (2), the thermoplastic resin powder is simply semi-melted and deposited on the reinforcing fiber bundle. Therefore, the dissociation of the thermoplastic resin powder by an external force cannot be completely protected against because the thermoplastic resin powder does not constrain the reinforcing fiber bundle. Additionally, the reinforcing fiber bundle is readily split by an external force, sometimes causing problems in the handleability of the molding material. Since the molding material is so bulky as described in (1), furthermore, similar problems occur.
According to the method (3), the reinforcing fiber bundle is fastened tight via the fiber of the thermoplastic resin. Therefore, the method (3) can overcome the splitting problem of the resulting molding material better than the method (2), but the production process then gets more complex. By the method (3), furthermore, the dissociation of the thermoplastic resin powder deposited onto the reinforcing fiber bundle cannot be protected against satisfactorily. Since the molding material is so bulky as described in connection with (1), additionally, similar problems occur.
Because the molding material produced by the method (4) contains an aqueous medium at a concentration as high as 10 to 70% by weight, a process of evaporating the aqueous medium is essential to molding the material. Therefore, the method is problematic in that the molding process is complex and in that adverse effects on the molded articles, such as the occurrence of voids, can hardly be avoided due to a high content of the evaporating components.
So as to avoid these problems caused by the higher concentration of the aqueous medium in the molding material produced by the method (4), the molding material is dried to evaporate the aqueous medium to below 10% by weight. But such a method is also disadvantageous in that the drape deteriorates or is eliminated in the resulting material. The deterioration of the drape through the evaporation of the aqueous medium is presumed to be due to a mechanism in production of the molding material by the method (4), by which the reinforcing fiber bundle and the thermoplastic resin powder are impregnated with the aqueous solution of the water-soluble polymeric binder of a high viscosity and the water-soluble polymeric binder works to more rigidly bind the reinforcing fiber bundle and the thermoplastic resin powder together, upon evaporation of the aqueous medium.
Because the method (4) applies a highly viscous dispersion solution of 50,000 cps to 3,000,000 cps, comprising a water-soluble polymeric binder, an aqueous medium and a thermoplastic resin powder to a reinforcing fiber bundle, the method has a drawback in that uniform dispersion of the thermoplastic resin powder is difficult.
According to the method (4), it is recommended to add a surfactant for the purpose of uniformly dispersing the thermoplastic resin powder in an aqueous medium or to add an ammonium compound for the purpose of imparting adhesiveness to the molding material. However, the thermal decomposition temperature of these compounds is lower than the molding temperature of thermoplastic matrix resins in general use in industry. Thus, these compounds may be thermally decomposed and evaporated during molding, which may disadvantageously affect the mechanical performance of the resulting molded articles or the appearance of the molded articles.
Because all the water-soluble polymeric binders to be used according to the method (4) have lower thermal decomposition temperatures of about 200.degree. C. to 250.degree. C., the water-soluble polymeric binders are decomposed disadvantageously at the molding temperature of thermoplastic matrix resins in general industrial use, when the molding materials produced by the method are directly used for molding. Thus, adverse affects on the appearance and physical properties of the resulting molded articles may occur. Furthermore, method (4) is disadvantageous in that the molding process is more complex, because decomposition of the components decomposable at the molding temperature, such as the water-soluble polymeric binders, surfactant, ammonium compounds and the like in the molding material, is inevitable prior to or during molding.
The processes described in the aforementioned individual gazettes concerning method (4) have one objective to produce a molding material imparted with adhesiveness. The molding material thus imparted with adhesiveness is advantageous in that hardly any slippage in fiber orientation occurs in laminating the material together because the individual sheets of the material adhere to each other. Further, the material is particularly suitable for sheet winding because the material readily adheres to a core bar when wound up over the bar. One drawback of imparting adhesiveness is the inclusion of a solvent and the like and a process of evaporating the solvent and the like is therefore essential during a molding process as described above. Contrary to the advantage described above, the imparted adhesiveness per se causes laborious work in handling the resulting molding material. For example, the materials stick onto molds or stick to each other during a lay-up operation or the reinforcing fiber eventually deforms when the molding material, once attached, is pulled off and adjusted.
It is an object of the present invention to provide a molding material wherein a thermoplastic resin powder is fixed in a reinforcing continuous fiber bundle via a binder resin and a method for producing the material. More specifically, it is an object of the present invention to provide a molding material providing excellent characteristic features in molded articles as described in the following item (i) as well as excellent handleability as described in the following item (ii); and a method for producing such a material.
In other words, the objects of the present invention reside in providing:
(i) a molding material capable of giving good composite properties and appearance to molded articles, and a method for producing the molding material, wherein a dispersion of a highly dispersible thermoplastic resin powder can be applied to a reinforcing continous fiber bundle, whereby the thermoplastic resin powder can be uniformly applied into between the individual fibers of the reinforcing continuous fiber bundle, and additionally the fiber content in the molding material can be increased; and concurrently
(ii) a molding material with good handleability, which is non-adhesive and does not contain substantially any solvent for the binder resin, which is drapable, which is not susceptible to the dissociation of the thermoplastic resin powder or the splitting of the reinforcing fiber and which is improved in terms of bulkiness.
In addition to the objects (i) and (ii), another object of the present invention is to provide a molding material characterized in that the decomposition of a binder resin contained in the molding material can be suppressed during molding and that gas generation is negligible, and a method for producing such a molding material.